[time-nuts] FE-5680A performance - 1 PPS output issue

Chris Albertson albertson.chris at gmail.com
Thu Jan 5 18:17:32 UTC 2012


On Thu, Jan 5, 2012 at 9:46 AM, beale <beale at bealecorner.com> wrote:

> I think there is something funny about the 1 PPS output on pin 6 from the
> currently available cheap FE-5680A units. I have three of these units. On
> one unit, on one occasion, I did observe a logic-level 1 PPS pulse, exactly
> 1 microsecond wide.  But after a power cycle it never came back, although
> the unit indicates a locked condition, and is apparently working. The other
> two units also seem to work (10 MHz output OK, lock OK) but I have never
> seen a 1 PPS signal on pin 6 from them. My Tek TDS-210 is easily capable of
> triggering on and displaying a 1 usec pulse.  Is it possible the pulse
> appears only after a RS-232 command, or after some other special condition?


I think the signal is there, it is just very small

I tried again this morning.  On mine the pulse is there but it's very weak.
  I can't see it one the scope but I can see the trigger light flash every
second if I work to set it "just right".  I also see an about 0.3V peak to
peak, about 60MHz sine wave.   I can get the scope to trigger and display
the sine wave just fine.  I have to use a 10X scope probe or I load the
signal to "millivolts"

The PPS signal coming from my Oncore GPS is easy to see on my scope and
even on a DMM set to "DC Volts"

The FE-5680's PPS is not usable directly with my counter, I have to amplify
it then set the counter to 10X attenuation then mess with the trigger
setting so it does not see the 60MHz signal.     Maybe I'm lucky that my
scope has a BNC output on the back for the vertical amplifier, so I can
apply a 20MHz low pass filter and about 20X voltage gain.

The documentation I have says pin 6 is "N/C" but it looks like there is a
way to extract a usable PPS but I think I'm going to need and LC filter,
some op amps and a one-shot and a TTL level driver.   My guess is that pin
6 is either some kind of engineering test/diagnostic signal not intended to
use used or the signal is an accident




>
> >  -------Original Message-------
> >  From: David <davidwhess at gmail.com>
> >  To: Discussion of precise time and frequency measurement <
> time-nuts at febo.com>
> >  Subject: Re: [time-nuts] FE-5680A performance
> >  Sent: 05 Jan '12 07:49
> >
> >  On Thu, 05 Jan 2012 01:44:13 -0800, Hal Murray
> >  <hmurray at megapathdsl.net> wrote:
> >
> >  >albertson.chris at gmail.com said:
> >  >>
> >  >> Have an older Tek 465 scope that is in only "fair" shape and I see
> nothing
> >  >> on that pin but milivolt level sine wave of about 60MHz.  I can't
> set the
> >  >> scope to show any hint of a PPS ...
> >  >
> >  >I do have a 465.  You should be able to see a 1 uSec PPS.
> >
> >  60MHz is about a 6nS rise time and is easily fast enough to see it.
> >
> >  >Now turn up the Intensity until you can see the pulse.  It might help
> to turn
> >  >down the room lights.
> >
> >  This is the problem.  With a 1 second repetition rate, the brightness
> >  is going to be very low.
> >
> >  The old ways of viewing such a low repetition rate signal include
> >  using a hood or dark room, special CRT phosphors, photographic film,
> >  MCP (micro channel plate) intensified CRTs, and of course analog
> >  storage and later digital storage.
> >
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> >
>
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-- 

Chris Albertson
Redondo Beach, California


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